Preheat the oven to 180°C (350°F) and line a 9'x9' baking tin with baking paper.
Melt the butter and dark chocolate together.
Mix the eggs, sugar, brown sugar and vanilla extract together until the mixture becomes a light golden colour.
Mix in the chocolate mixture.
Stir in the flour and cocoa until just combined.
Spread half the mixture into the prepared pan in an even layer.
Break off four fingers of Kitkats and place the remaining Kitkats on top of the brownie mixture.
Top with the remaining brownie batter.
Break up the four KitKat fingers and spread over the top and place in the oven to bake for 20-25 minutes. The top and sides will be set and the middle will have crumbs attached to a skewer inserted but should not be wet.